Washington DC - Democratic candidates for US president, meeting in their first debate on Tuesday, clashed over foreign policy and the best use of American military power in the escalating conflict in Syria.

Hillary Clinton, former secretary of state and favourite for the Democratic nomination, said the United States should take a more assertive role in Syria by creating a safe zone to protect civilians and send a forceful message to Russian President Vladimir Putin.

"It's important that the United States make it very clear to Putin that it's not acceptable for him to be in Syria creating more chaos, bombing people on behalf of [President Bashar] Assad, and we can't do that if we don't take more of a leadership position," Clinton said in the debate.

While she praised President Barack Obama for engaging in talks with the Russians on Syria, her advocacy of a stronger US policy would involve greater military intervention and drew criticism from her leading challenger, Senator Bernie Sanders.

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"Let's understand that when we talk about Syria, you're talking about a quagmire in a quagmire," Sanders responded. "We should be putting together a coalition of Arab countries who should be leading the effort."

Sanders said he supports Obama's effort "to thread a tough needle here" by opposing Assad and simultaneously fighting the Islamic State of Iraq and the Levant (ISIL) without putting US forces into combat.

"I do not support American ground troops in Syria", and imposing a no-fly zone would create "a very dangerous situation", Sanders said.

Former Maryland Governor Martin O'Malley said Clinton is too quick to resort to the use of military force. "I would not be so quick to pull for a military tool," O'Malley said. Addressing Clinton, "a no-fly zone in Syria, at this time, actually, Secretary, would be a mistake", he added.

Clinton has the major financial and political backing of the Democrat establishment in the US, but Sanders has been drawing large, enthusiastic crowds of 10,000 to 20,000 to rallies, raising significant funding from small donors, and is tied with Clinton in polls in Iowa and leading New Hampshire, two early primary states.

The debate over Syria matters because as the crisis worsens, how the US presidential candidates would handle US policy in the Middle East region will be a critical question going into the 2016 general election.

So far, Obama has declared the US will not get into a proxy war with Russia and is pursuing talks with Moscow to create safe zones for civilians and initiate negotiations towards a political resolution of the conflict.

Republican candidates have painted Obama as weak and ineffective in foreign affairs and have pledged to use the US military more aggressively overseas, including putting boots on the ground to fight ISIL and promising to reverse the Iran nuclear agreement.

"We have a president who doesn't have a clue," Republican front-runner Donald Trump said in the first Republican debate in August. "I would say he's incompetent, but I don't want to do that because that's not nice."

Jeb Bush has said he would cancel the nuclear deal with Iran and expand US troop presence in Eastern Europe to deal with Putin, who he called a "bully".

Marco Rubio has charted a substantive foreign policy course for himself. A member of the Senate Foreign Relations Committee, he penned a manifestoin an influential magazine arguing for a more robust assertion of American power on the world stage.

The American electorate is divided along partisan lines about US foreign policy and national security.

"The rise of ISIS has had clear impact on attitudes across the board, but especially among Republicans," said Carroll Doherty from the Pew Research Center, a nonpartisan organisation in Washington that surveys American attitudes and how events shape public opinion.

In March 2014, 57 percent of Republicans said the US should use overwhelming military force to deal with "terrorism".

In February 2015, 74 percent of Republicans supported military action, a 17 percentage-point increase in less than a year.

In both surveys, two-thirds of Democrats said reliance on military force creates hatred and engenders more violence.

"This is a fundamental point of disagreement between Republicans and Democrats: how to address terrorism and the threat of terrorism around the world," Doherty told Al Jazeera.

Americans have yet to fully digest the import of events in Syria even though a 55-percent majority say they were hearing a lot about the crisis at the end of September.

According to a Pew survey, the public reaction followed partisan lines: 67 percent of Republicans said "no" to the US accepting more refugees, and 69 percent of Democrats said "yes".

In the short term, Europeans expect the US to do what's feasible and that is to provide more financial support to the struggling UN food aid and refugee assistance programmes, and to accept more people than it has so far, said Matteo Garavoglia, a fellow at the Brookings Institution in Washington.

The US state department plans to increase its quota of Syrian refugees from 1,500 to 10,000 next year, subject to security screening and background checks. That compares to the 800,000 Syrian refugees Germany will accept.

The US has pledged $1.5bn in funding to UN programmes for Syria this year and Congress is advancing a spending bill to provide more.

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"All this falls into a far broader issue. The question is what to do and nobody has the answer yet," Garavoglia told Al Jazeera. "There is a huge debate within the policy community here in the US and there is no consensus."

Whoever is elected president will face a difficult reality of limited US leverage and bad options in the Middle East when they take office in 2017, said Aaron David Miller, a scholar at the Wilson Center in Washington.

Anything the candidates say now cannot be seen as determinative about what they'll do once in office, he said.

For now, the presidential candidate debates in the US are aimed at core party primary voters who tend to hold more sharply defined views. The tone and quality of debate will shift when the two major parties nominate their candidates and the contest moves towards the decisive 2016 general election.

"Other than terrorism and appearing to stand tall, I'm not sure most Americans are all that interested in what Vladimir Putin is doing in Syria, or whether Egypt is going to turn into a democracy," Miller told Al Jazeera.

"When most Americans look at presidents and foreign policy, they are thinking: 'Is this person commanding, poised? Is this person going to make America look good? Is this person tough - but do they have wisdom, prudence, and does he or she know what she's talking about it?''' Miller said.